1. Field of the Invention
The present invention relates to a code division multiple access (CDMA) communication system, and more particularly, to an apparatus and method for receiving a channel signal transmitted using a space time transmit diversity (STTD) scheme.
2. Description of the Related Art
As mobile telecommunication systems have rapidly developed and the amount of data for use in such mobile telecommunication systems has rapidly increased, the third generation mobile telecommunication system for transmission of data at a higher speed has been recently developed. As for the third generation mobile telecommunication system, a W-CDMA (Wideband-Code Division Multiple Access) scheme, which is an asynchronous scheme among Node Bs, has been widely used in Europe as the wireless access standard, and a CDMA-2000 scheme, which is a synchronous scheme among Node Bs, has been widely used in North America as the wireless access standard. Typically, the mobile telecommunication system enables a plurality of user equipments (UEs) to intercommunicate via one Node B. However, phase distortion of the reception signal occurs in the mobile telecommunication system because the fading phenomenon occurs on the wireless channel during transmission of high-speed data. The fading phenomenon causes the amplitude of a reception signal to be reduced from several tens of dB to a few dB. Therefore, if the distorted phase of the reception signal is not compensated during a data demodulation process, an undesirable information error occurs in transmission data from the transmission end, such that the quality of service (QoS) of the mobile telecommunication system is deteriorated. In order to transmit high-speed data without such QoS deterioration, the problem of the fading phenomenon must be solved. The result is that a variety of diversity schemes have been widely used to solve such fading phenomenon.
Typically, a CDMA scheme uses a rake receiver for performing diversity reception using delay spread of a channel signal. A general rake receiver uses a receive diversity scheme for receiving a multi-path signal. However, a rake receiver based on a diversity scheme using the delay spread is inactivated when a value of the delay spread is lower than a prescribed value. Also, a time diversity scheme using an interleaving and coding operation is typically used for a Doppler spread channel. However, it is difficult to use the time diversity scheme in a low-speed Doppler spread channel.
Therefore, in order to solve the fading phenomenon, a space diversity scheme has been used for a channel having both a low delay spread, such as an indoor channel, and a channel having a low-speed Doppler spread, such as a walker channel. The space diversity scheme uses at least two transmission/reception antennas. Described in greater detail, if the magnitude of a signal transmitted via one antenna is reduced by the fading phenomenon, the space diversity scheme demodulates a transmission signal by receiving signals transmitted via the rest of antennas. The space diversity scheme is classified as a receive antenna diversity scheme using a reception antenna, and a transmit antenna diversity scheme using a transmission antenna. However, because the receive antenna diversity scheme is used for UEs, it is difficult to install a plurality of antennas to each UE in light of the size and cost of each UE. Therefore, it is preferable to use the transmit antenna diversity scheme, which installs many antennas at a Node B.
The transmit antenna diversity scheme uses a specific algorithm for receiving a downlink signal to obtain a diversity gain, the diversity gain can be classified as an open loop mode transmit diversity and a closed loop mode transmit diversity. In case of the open loop mode transmit diversity, wherein the Node B encodes information bits and transmits them via a plurality of diversity antennas, the UE receives signals transmitted from the Node B and decodes the received signals such that a certain diversity gain is obtained. In case of the closed loop mode transmit diversity, wherein the UE estimates and calculates channel environments through which signals transmitted via transmission antennas of a Node B will travel in the future, weighted values of the antennas of the Node B are calculated on the basis of the calculated estimation values in order to obtain a maximal power value of a reception signal. The weighted values are then transmitted to the Node B via an uplink, wherein the Node B receives them from each UE and applies each of the weighted values to each antenna, thereby adjusting each weighted value of the antennas. In this case, the Node B transmits a pilot signal to every antenna to measure a channel of the UE, resulting in the UE measuring the channel using the pilot signal for every antenna and finds an optimal weighted value based on the measured channel information.
The channel of a mobile telecommunication system using the W-CDMA scheme is mainly comprised of a physical channel, a transport channel, and a logical channel. The physical channel can be further classified as a downlink physical channel and an uplink physical channel according to the transmission direction of information data. The downlink physical channel can then be further classified as a physical downlink shared channel (PDSCH) and a downlink dedicated physical channel (DPCH), and will hereinafter be described with reference to FIG. 1.
FIG. 1 is a view illustrating a configuration of a downlink DPCH of a mobile telecommunication system.
Referring to FIG. 1, each frame of the downlink DPCH includes 15 slots (slot#0 through slot#14). Each slot includes a DPDCH dedicated physical data channel (DPDCH) for transmitting upper layer data transmitted from a Node B to a UE, and a dedicated physical control channel (DPCCH) for transmitting a control signal of a physical layer. The DPCCH includes a transmit power control (TPC) symbol for controlling the transmission power of the UE, a transport format combination indicator (TFCI) symbol, and a pilot symbol. As shown in FIG. 1, each slot contained in one frame of the downlink DPCH is comprised of 2560 chips (1 chip=1 bit). A Data1 symbol and a Data2 symbol each indicate upper layer data transmitted from the Node B to the UE over the DPDCH. The TPC symbol indicates information for enabling the Node B to control a transmit power (TP) value of the UE. In the meantime, the TFCI symbol indicates which one of transport format combinations (TFCs) is applied to a downlink channel transmitted during one frame of 10 ms duration. Finally, the pilot symbol indicates a reference for enabling the UE to control a TP of a DPCH. In this case, information contained in the TFCI symbol is classified into dynamic part information and semi-static part information. The dynamic part includes transport block size (TBS) information and transport block set size (TBSS) information. The semi-static part includes transmit time interval (TTI) information, channel coding scheme information, coding rate information, static rate matching information, and cyclic redundancy check (CRC) size information, etc. Therefore, the TFCI symbol indicates the number of transport blocks (TBs) of a channel transmitted during one frame, and assigns a predetermined number to a TFC available in each TB.
A signal transmission process over the DPCH uses a space time block coding based transmit diversity (STTD) scheme among the aforementioned open loop mode transmit diversity schemes, as prescribed in the UMTS standard TS 25.211. Besides the DPCH, there are a variety of channels adapting the STTD scheme, i.e., a P_CCPCH primary _common control physical channel (P_CCPCH), a secondary_common control physical channel (S_CCPCH), a synchronization channel (SCH), a page indication channel (PICH), an acquisition indication channel (AICH), and a PDSCH, among others.
A channel encoding operation according to the STTD scheme will hereinafter be described with reference to FIG. 2.
FIG. 2 is a view illustrating a channel encoding process using a STTD encoder.
Referring to FIG. 2, a plurality of symbols sequentially enter a STTD encoder 119 according to a transmit diversity coding section used in the transmit diversity scheme. The STTD encoder 119 encodes the symbols with the STTD scheme, and then outputs the encoded symbols to two transmission antennas, i.e., a first antenna and a second antenna. For example, if a symbol S1 enters a transmit diversity coding section T1 and then a symbol S2 enters a transmit diversity coding section T2, (i.e., if the symbols S1˜S2 sequentially enter the STTD encoder 119), the STTD encoder 119 performs the STTD encoding on the symbols S1˜S2, transmits a symbol signal S1S2 to the first antenna, and transmits a symbol signal −S2*S1* to the second antenna.
A channel information bit encoding operation of the STTD encoder 119 shown in FIG. 2 will hereinafter be described with reference to FIG. 3.
FIG. 3 is a view illustrating a channel information bit encoding process via the STTD encoder 119 shown in FIG. 2.
Referring to FIG. 3, it is assumed that the symbols S1˜S2 sequentially received according to the transmit diversity coding sections are composed of channel information bits of b0b1 and channel information bits of b2b3, respectively. Initially, channel information bits b0b1b2b3 corresponding to the symbols S1˜S2 enter the STTD encoder 119. The STTD encoder 119 performs the STTD encoding on the channel information bits of b0b1b2b3, thereby transmitting channel information bits b0b1b2b3 (S1S2) to the first antenna, and transmitting channel information bits -b2b3b0-b1 (-S2*S1*) to the second antenna.
As described above, the downlink DPCH signals are transmitted according to the STTD encoding scheme. Particularly, signals transmitted over the TPC field and the pilot field of the downlink DPCH are STTD-encoded according to the following regulations.
The TPC field's signals, i.e., TPC bits, are all STTD-encoded. Typically, signals over the TPC field are transmitted with bits having the same value during one slot time. In this case, if the STTD encoding is applied to the TPC field and the number of TPC bits transmitted over the TPC field is 4 or 8, the STTD encoding among the TPC bits is performed. In the meantime, because the STTD encoding is not performed using TPC bits only when the number of the TPC bits is 2, the TPC bits are STTD-encoded along with the last two bits of the Data1 field and then transmitted to the first and second antennas.
The pilot field is differently encoded differently according to the number of its own bits. For example, if the number of bits of the pilot field is 2, the STTD encoding is performed along with the last symbol of the Data2 field. If the number of bits of the pilot field is 4, the STTD encoding is performed between two symbols of the pilot fields. If the number of bits of the pilot field is 8 or 16, i.e., a multiple of 8, the STTD encoding is performed between even symbols and an encoding operation is performed between odd symbols to maintain orthogonality of the signals of the pilot field. Here, the pilot field forms one symbol with two bits such that one pilot symbol is formed when the number of bits of the pilot field (hereinafter referred to as pilot bits) is 2. So, the STTD encoding is performed on the formed one pilot symbol along with the last symbol of the Data2 field adjacent to the pilot field. Also, two pilot symbols are formed when the number of pilot bits is 4, such that the STTD encoding between the formed two pilot symbols is performed. At least four pilot symbols are formed when the number of pilot bits is at least 8, such that the STTD encoding between even pilot symbols is performed, and an encoding operation is performed between odd pilot symbols to maintain orthogonality of signals of the pilot field. In this way, if the symbols of the pilot field are STTD-encoded, orthogonality is provided to signals of the pilot fields of a plurality of antennas, i.e., first and second antennas.
Tables 1 and 2 below describe the signal patterns (i.e., pilot patterns) transmitted over the pilot field.
TABLE 1Npilot =Npilot = 4Npilot = 8Npilot = 16Symbol2(*1)(*2)(*3)#001012301234567Slot #0111111111111101111111011111110 1001100110011101100111011111100 2011101110111011101110111101100 3001100110011001100110011011110 4101110111011011110110111111111 5111111111111101111111011011101 6111111111111001111110011101111 7101110111011001110110011101100 8011101110111101101111011001111 91111111111111111111111110011111001110111011101110111011111111011101110111011111110111111001110121011101110110011101100110111011300110011001111110011111100110014001100110011111100111111101101
Table 1 above shows pilot patterns transmitted over the first antenna, and her pilot patterns transmitted over the second antenna are shown in Table 2.
TABLE 2Npilot = 2Npilot = 4Npilot = 8Npilot = 16Npilot = 4Symbol(*1)(*2)(*3)(*4)(*5)#00101230123456701Slot #00101101100001011000010110000100110 11010101100000111000001111000101001 21111101111000011110000111000111100 31010101110000111100001110000001001 40000101111001111110011110100100011 50101101100001011000010111100000110 60101101110001011100010110100110110 70000101110001111100011111000110011 81111101100000011000000110100011100 90101101101001011010010110100010110101111101111000011110000110000101100110000101101001111010011110000010011120000101110001111100011111100000011131010101101000111010001111000011001141010101101000111010001111100111001
The most important factor for controlling overall performance of a mobile telecommunication system using the W-CDMA scheme is the power control (PC) function of the DPCH. Therefore, rapid TPC transmit power control (TPC) is generally needed, as prescribed in the UMTS standard TS 25.211& TS 25.214. In order to increase the number of UEs a Node B can handle (i.e., the accommodation capability), a transmission signal from the Node B is preferably maintained at or greater than a predetermined threshold value at each reception end of the UEs, and signals transmitted from each UE should not be affected by interference. Therefore, the UE adjusts its own TP using a TPC symbol received over a TPC field of a downlink DPCCH transmitted from the Node B, calculates a signal to interference ratio (SIR) of the reception signal using pilot symbols received over the pilot field, produces TPC information of the Node B using the calculated SIR, and reports the TPC information to the Node B in such a way that the Node B adjusts a TP value of the UE.
A power control (PC) timing of the DPCH will hereinafter be described with reference to FIG. 4.
FIG. 4 is a view illustrating a PC timing diagram of the DPCH.
Referring to FIG. 4, if a UMTS terrestrial radio access network (UTRAN) transmits a downlink DPCCH signal (DL_DPCCH at UTRAN), the UE experiences a propagation delay for a predetermined time before receiving the downlink DPCCH signal (DL_DPCCH at UTRAN) from the UTRAN. The UE receiving the downlink DPCCH signal from the UTRAN reads out the TPC field of the downlink DPCCH, and modifies its TP (Transmit Power) value according to the corresponding received TPC command. The UE reads out the pilot field of the downlink DPCCH to calculate the SIR of the reception signal, determines on the basis of the calculated SIR whether or not the TP of the UTRAN should be adjusted, and transmits the determination result to the UTRAN over a TPC field of an uplink DPCCH (UL_DPCCH at UE). In this case, the UE transmits the uplink DPCCH signal (UL_DPCCH at UE) to the UTRAN at the controlled TP value. The UTRAN receives the uplink DPCCH signal (UL_DPCCH at UE) from the UE after a lapse of a predetermined time, i.e., due to propagation delay of the uplink DPCCH signal (UL_DPCCH at UE). The uplink DPCCH signal (UL_DPCCH at UE) contains the UTRAN's TPC information calculated by the UE. However, in order to meet the 1-slot delay PC (Power Control) regulation prescribed in the 3GPP (3rd Generation Partnership Project) while performing the above DPCH PC function, the total delay time should be within a specified time limit of 512 chips. This specified time limit takes into account the multi-path delay time and process delay time for performance of the PC function on the basis of an antenna end. Therefore, if the total delay time is longer than the specified time of 512 chips, the 1-slot delay PC becomes impossible, resulting in deterioration of system performance.
Signals transmitted over the pilot and TPC fields of the DPCCH used for the downlink DPCH PC are STTD-encoded as described above, such that the UE must perform the STTD encoding when demodulating the signals transmitted over the pilot and TPC fields of the DPCCH. An apparatus for demodulating the STTD-encoded DPCCH signals will hereinafter be described with reference to FIG. 5.
FIG. 5 is a view illustrating an internal configuration of a conventional apparatus for demodulating a STTD-encoded DPCH signal.
Referring to FIG. 5, if the DPCH signal is STTD-encoded and transmitted as described above, the STTD-encoded DPCH signal is received over a UE antenna (not shown), and then the received DPCH signal enters an STTD decoder 511. The STTD decoder 511 receives the DPCH signal, decodes it with the STTD decoding scheme corresponding to the STTD encoding scheme used in the UTRAN, and then generates the decoded DPCH signal. The STTD-decoded DPCH signal generated from the STTD decoder 511 enters a data demodulator and a DPCCH processor 513. The DPCCH processor 513 receives the DPCCH signal among the STTD-decoded DPCH signals as an input, and processes TPC and pilot symbols transmitted over the TPC and pilot fields. The TPC and pilot symbols generated from the DPCCH processor 513 are considered as measurement values of the DPCH signal. As a result, the UE recognizes a SIR value of a signal received over the DPCH on the basis of the pilot symbol, and recognizes its own TP control value on the basis of the TPC symbol. Thereafter, the UE generates a TPC command to be transmitted over the TPC field of the uplink DPCCH, and transmits the TPC command to the UTRAN over the TPC field such that a TPC process is correctly established. A timing diagram for the DPCH signal demodulation process will hereinafter be described with reference to FIG. 6.
FIG. 6 is a view illustrating a timing diagram for the DPCH signal demodulation process shown in FIG. 5.
For the convenience of description and better understanding of the present invention, a pilot signal received over the pilot field among a plurality of DPCH signals is adapted as an example in reference to FIG. 6. Referring to FIG. 6, a signal received over the pilot field, i.e., a pilot signal, enters the STTD decoder 511 at a certain time, for example, at time t=0 which is shown on the time axis (t). The pilot signal is STTD-decoded by the STTD decoder 511 and outputted at a certain time, for example, at time t1, also shown on the time axis. The SIR value of the received DPCH signal is calculated using the STTD-decoded pilot signal. A TPC command for executing a PC process of the UE is produced using the calculated SIR value at a certain time, for example, at time t2, also shown on the time axis. As can be seen from FIG. 6, the STTD decoding unavoidably causes a delay time of t1.
It can be seen that, therefore, given that the STTD decoding is adapted to process signals transmitted over the pilot and TPC fields of the DPCH, the total delay time becomes longer in the PC process, resulting in deterioration of system performance.